p0303 misfire... scratching my head

wackinwallaby

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Mar 11, 2019
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I have a 2000 mustang gt that has 93k miles. It is throwing a p0303 misfire code. The engine is running rough, but only when it is cold. I can only notice the misfire and rough running when it is at idle and cold. After it is warmed up to operating temps, it runs great. Items I have tried so far:
  1. All new plugs installed(Motorcraft plugs). no fix
  2. Moved cyl 3 cop to position 1, still threw cyl 3 code (has aftermarket accell COP's on it)
  3. Moved cyl 3 injector to position 1, threw cyl 3 code still
  4. cleaned all connectors under the hood that i could get to with contact cleaner(including the COP and injector connectors)
  5. checked wire bundles, found a wire bundle that had the insulation worn off but no bare wires showing, repaired it, no fix
  6. cleaned MAF sensor and intake duct cleaned (also has aftermarket air intake, greenfilter)
  7. replaced Intake air control valve no fix
  8. tried different octanes, 93 and 87, i know its supposed to have 87 octane per the manual, but just to try something i put a tank of 93
  9. Ran injector cleaner through the tank
  10. Ran load test on alternator and seems to be fine (stays above 13v when loaded and lights dont flicker or anything to indicate a bad diode)
  11. Just a note: they replaced the battery just before I purchased the car(have had the car for about 2 months)
Not sure where to go from here. Any ideas or suggestions would be greatly appreciated.
Thanks all!
 
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wmburns

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SO If you are certain that the battery charging system is in tip top shape and the ignition is also good this cuts down the options to things like:
  • Clogged injector(s)
  • base engine problem such as low compression
  • base engine problem such as unstable timing.
For a suspected base engine problem a compression and cylinder leak down test is recommended. The tool can be rented from most auto parts stores. The test itself is not hard to do.

For a possible fuel injector "issue" I have had great luck with injector cleaning and flow test services such as InjectorRX.com. Once cleaned and tested they are as good as new.

An option is to replace a single injector. Personally if I need to pull one, I'm going to pull them all and send the entire set out for cleaning. Be sure to record which cylinder each injector is removed from.
 

wackinwallaby

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Mar 11, 2019
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Thanks for the reply wmburns!
So I wanted to mention that I did swap out the cyl 3 injector with cyl 1 to try to rule out the injector and cyl 3 code still persists.

I will rent the compression check tool as soon as I can and check it out and repost.

Wanted to ask, do you think it is possible that the PCM is causing my problem. Possibly a bad electronic component on the pcb that would be out of tolerance when cold, but in tolerance when warm? Is it worth it to try to swap the PCM? Thanks again for the reply!
 

wackinwallaby

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Mar 11, 2019
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So last night I checked the signal to all of the COP's and injectors with my 12v light. All COP's and injectors blinked like they are supposed to. To me, that rules out the PCM and wiring... So even though the 12v is getting to the COP and injector, cyl 3 is still not firing when cold. Next is compression. I picked up a compression check tool from harbor freight and will check the compression this weekend. Also will try to look around in the cylinders with a bore scope, hopefully i can see something... Will repost with updates after this weekend...
 

wackinwallaby

Member
Mar 11, 2019
12
1
13
33
Ohio, US
I have a 2000 mustang gt that has 93k miles. It is throwing a p0303 misfire code. The engine is running rough, but only when it is cold. I can only notice the misfire and rough running when it is at idle and cold. After it is warmed up to operating temps, it runs great. Items I have tried so far:
  1. All new plugs installed(Motorcraft plugs). no fix
  2. Moved cyl 3 cop to position 1, still threw cyl 3 code (has aftermarket accell COP's on it)
  3. Moved cyl 3 injector to position 1, threw cyl 3 code still
  4. cleaned all connectors under the hood that i could get to with contact cleaner(including the COP and injector connectors)
  5. checked wire bundles, found a wire bundle that had the insulation worn off but no bare wires showing, repaired it, no fix
  6. cleaned MAF sensor and intake duct cleaned (also has aftermarket air intake, greenfilter)
  7. replaced Intake air control valve no fix
  8. tried different octanes, 93 and 87, i know its supposed to have 87 octane per the manual, but just to try something i put a tank of 93
  9. Ran injector cleaner through the tank
  10. Ran load test on alternator and seems to be fine (stays above 13v when loaded and lights dont flicker or anything to indicate a bad diode)
  11. Just a note: they replaced the battery just before I purchased the car(have had the car for about 2 months)
Not sure where to go from here. Any ideas or suggestions would be greatly appreciated.
Thanks all!
To add to #1 - forgot to mention that the previous owner had a different style plug in cyl 3(a high performance one as opposed to standard ones in all the other cylinders), so that tells me he knew the car had a problem and tried to fix it. I asked him about the problem and he denied any knowledge of course....
 

wackinwallaby

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Mar 11, 2019
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Ohio, US
Compression check complete,

Cyl 1- 195
Cyl 2 - 190
Cyl 3 - 40
Cyl 4 - 165
Cyl 5 -190
Cyl 6 - 195
Cyl 7 - 190
Cyl 8 - 195

Well I definitely have an issue with cyl 3... I guess a leak down test is next to try to determine where the problem lies. Do you think that Cyl 4 is suspect bad as well with slightly lower than the rest?
 

wackinwallaby

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Mar 11, 2019
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Ok have more info...

Did a wet compression test to rule out piston rings and compression only rose about 10 psi so I don’t think I have bad rings...

I warmed up the motor and out of curiosity did a compression test and got 110-115psi on cylinder 3. So something is sealing better when warm.

Did a leak down test with cylinder 3 at tdc for the combustion stroke and the air was rushing out of the intake. So it looks like something with the intake valve is causing the issue.

I was thinking that maybe it is gummed up with carbon and so I ran some Seafoam through it and while it didn’t fix the problem as of yet, that side of the motor is really dirty. After I pulled back in the garage it left speckles of carbon on my garage floor(attached pictures) just from idling but only on the side I’m having issues... my question now is do you think it is carbon buildup that is causing my problem? Should I run Seafoam a few more times to try to fix or should I pop the valve cover and take a peak? Any advice?
286B673C-28BE-418F-A916-5F814F55D262.jpeg
824C0671-0184-4A2B-95A2-24280FB13A63.jpeg
 

wmburns

SN Certified Technician
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If this were my car I would consider renting/buying a USB borescope to take a look inside the spark plug holes.

It would be handy to confirm IF carbon deposit is the reason for the low compression.

Another option is to take the valve covers off and inspect the roller follows. However I suspect this won't yield any useful information.
 

wackinwallaby

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Mar 11, 2019
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I have a borescope and there is a decent amount of carbon buildup on the piston heads... is there a good way to see the valves with the camera? I thought about taping it to its own wire so I could look up there but I’m just afraid of getting it stuck.. suppose I could try. I would just need to make sure the intake valve is open... I’ll attach some pics of the piston heads pre Seafoam
 

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wmburns

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I am currently dealing with a similar issue on a GM. I believe that I have a bad coil in a coil pack waste spark ignition system. For this GM 3.5 cylinders #2 and #5 share a coil. The weak coil caused the spark plugs to read RICH. The other cylinders looked perfect. #2 & #5 did have carbon build up on the pistons. The scope I used has a 45 degree mirror that allows some view of the intake valve. Not perfect but better than nothing.

In my case I could see the carbon around the seal of the exhaust valve when it's open. I believe this is causing my compression test to read low.

The coil has been replaced and I'm trying to formulate a plan to clean the carbon deposit. However my case did not have near as much carbon as what you have.

IMO there's a real possibility that your low compression is from carbon build up.
 
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wackinwallaby

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Mar 11, 2019
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Ohio, US
I work with a guy who used to be an aircraft engine mechanic, he said that I could pull the valve cover and lightly tap the valve with a hammer with a piece of wood or something in between so you don’t damage the top of the valve and that may break the carbon free. I may try to treat with Seafoam a few more times to try to break it loose without taking anything apart. If I can’t get it loose I’ll pull the valve cover..

In the meantime I may try to get the camera in there to see if I can confirm that carbon is my problem, but I’m fairly certain that is the cause of my issue.

If you come up with any more ideas to clean the carbon off the valve please share with me. I’ll keep looking for ideas...
 

wmburns

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Are the intake valves accessible if i remove the intake manifold? May be worth it to clean them manually if i can remove the manifold and access them.
Yes you can almost all the way around the intake valve with the intake removed.

Given the amount of work required to remove an intake, have you considered something like:
View: https://www.amazon.com/OTC-7448A-Injection-Cleaner-Canister/dp/B00GZ0H9VC/ref=sr_1_30?keywords=intake+valve+cleaning+tool&qid=1557769574&s=gateway&sr=8-30


The advantage that a system like this has is:
  • It cleans the fuel injectors at the same time.
  • any deposits removed are immediately blown through the motor and out the exhaust.
  • It cleans the pistons.
  • It cleans the valves.
  • It is safer than "guessing" how much product can safely be sprayed into an intake.

If you do take the intake manifold off consider sending the full set of injectors out for cleaning/flow testing. I have had great luck with InjectorRX.com. Once cleaned and tested they are as good as new (but cheaper).
 

wackinwallaby

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Mar 11, 2019
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ok, forgive my ignorance, but i cannot find anything on the subject so I must ask. Is the 4.6 ford a GDI or port injection? If it is GDI it will not help the intake valves to use a fuel system cleaner such as that.

Im not sure what to do at this point, may just pull the valve cover and inspect to make sure nothing obvious is broken. If nothing is broken I would like to clean the intake valves.
 

wmburns

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ok, forgive my ignorance, but i cannot find anything on the subject so I must ask. Is the 4.6 ford a GDI or port injection? If it is GDI it will not help the intake valves to use a fuel system cleaner such as that.
Well..............How good would my advice be if a 2000 Mustang GT were a GDI? For the record. It's plain ole port injection.
 

wackinwallaby

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Mar 11, 2019
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Ok that’s an option then, sweet! Much better than tearing the manifold off...

So since it’s port injection, couldn’t I pull the injector and fuel rail off and look at the intake valve with my borescope through the injection port? Then I could verify that my problem is carbon or not. Assuming my camera will fit there... thanks for all your help!!!
 

wmburns

SN Certified Technician
Aug 14, 2009
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Yes. With a borescope you could see the back side of the intake valves through the injector hole.

Again if taking the fuel rail off anyway, consider having the entire set of injectors cleaned and flow tested. Be sure to record which cylinder each injector came from. Looking at the before report might help with the trouble shooting.
 

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